The present invention relates generally to the field of switched-mode power supplies and in particular to a switched-mode power supply having a variable minimum switching frequency.
Numerous mechanisms exist for implementing switched mode power supplies (SMPSs). The use of a switched inductor output circuit stands as a non-limiting example of one common implementation. In switched inductor topologies, the SMPS actively regulates its output voltage by switching the output inductor into and out of electrical connection with a supply voltage according to some form of regulation error feedback signal or other regulation control signal. When the inductor maintains continuous current as it is switched into and out of connection with the supply, the power supply is said to be operating in Continuous Conduction Mode (CCM).
SMPSs can improve their overall operating efficiencies by operating in “discontinuous conduction mode” (DCM) under certain load conditions. For example, a given SMPS may be configured to enter DCM responsive to detecting zero or negative current in the inductor. In other words, when the load current drawn from the SMPS falls to a light level, the SMPS may improve its overall operating efficiency by suspending active regulation switching operations, thus reducing switching losses. The SMPS thus effectively turns its switched output off and allows the load to “float.” CCM operation may be re-entered responsive to, e.g., the value of the output voltage.
Copending U.S. patent application Ser. No. 11/387,943, “Switched Mode Power Supply Method and Apparatus,” assigned to the assignee of the present application and incorporated by reference herein in its entirety, discloses a SMPS having “smart” output voltage regulation in DCM, whereby the floating load voltage is monitored, and regulated by entering CCM or activating an output voltage pull-down if the voltage exceeds one or more predetermined thresholds.
Under light loads, where the SMPS may operate primarily in DCM, entering CCM relatively infrequently, the effective switching frequency may be reduced to less than 20 kHz, which is within the range of human hearing (roughly 20 Hz-20 kHz). In some applications, the SMPS output switching may cause mechanical vibrations in one or more circuit components, generating objectionable audible noise. One way to eliminate this possibility is to restrict the minimum switching frequency to a value in the ultrasonic range, such as 25 kHz or above. However, this limits the efficiency benefits that can be achieved by DCM under light load conditions by operating at frequencies below ultrasonic.